core/pkg-generic: allow packages to exclude files when extracting

Currently, packages that need to exclude parts of the archives when
extracting (e.g. to gain space), like gcc or toolchain-external, have to
provide custom extract commands, just for the sake of adding a bunch of
--exclude directives when calling tar.

Add a new variable that packages may set, to provide a space-separated
list of patterns to exclude.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>